Field of the invention
This invention relates to a process for preparing synthetic zeolitic materials such as zeolite A and more particularly to a novel process for the preparation of zeolitic compositions which have improved ion exchange capacities.
Description of the prior art
Naturally occurring hydrated metal aluminum silicates are called zeolites and a great deal of patent and related art is concerned with materials of this type. In recent years, a large number of zeolites have been synthesized for use in various areas and significant differences are apparent between the natural and synthetic zeolites. A basic patent covering synthetic molecular sieves is U.S. Pat. No. 2,882,243 to Milton, issued Apr. 14, 1959, which is concerned with and describes a zeolitic material known as zeolite A. Zeolite A is a synthetic molecular sieve described in this patent as a three-dimensional crystalline framework of SiO.sub.4 and ALO.sub.4 tetrahedra which are cross-linked by the sharing of oxygen atoms in certain proportions. As described in the patent, zeolite A is distinguished from other zeolites and silicates on the basis of its X-ray powder diffraction pattern and is prepared from a sodium alumino silicate/water mixture having certain specific mole ratios and maintaining this mixture at 20.degree. - 175.degree. C. until crystals are formed and separating the crystals.
Since the recognition of zeolite A as a novel and useful zeolitic molecular sieve, other art has been concerned with methods for the preparation of this zeolite A material. Of these patents, of most interest to the present invention is U.S. Pat. No. 3,114,603 to Howell which discloses a process for the production of synthetic zeolite A from Kaolin-type materials by a process comprising digesting an aqueous reaction mixture containing the clay type materials. The digestion is carried out by contact of a calcined clay with a sodium hydroxide solution at temperatures of 20.degree. - 55.degree. C initially, followed by increasing the temperature to at least 75.degree. C under agitation for a sufficient period to crystallize the zeolite product.
A similar reaction is disclosed in U.S. Pat. No. 2,992,068 to Haden et al. Which describes a process for the production of crystalline zeolites from dehydrated kaolin clay with dehydration of the clay being carried out by calcination at temperatures of 800.degree. - 1600.degree. F to completely remove the water of crystallization from the clay. The dehydrated kaolin clay is then contacted with an aqueous solution of sodium hydroxide at a temperature in the range of 70.degree. - 115.degree. F until a homogeneous amorphous reaction product is formed. The resulting reaction product is then aged in the presence of water at a temperature of from 70.degree. - 325.degree. F to convert the amorphous product to a homogeneous polycrystalline zeolitic mass.
Similar processes are disclosed in U.S. Pat. Nos. 2,847,280 to Estes and 2,841,471 to Sensel, both of which disclose processes comprising contact of a clay-like material with an alkali metal solution such as sodium hydroxide, to form a zeolite A product under elevated temperature conditions.
The present invention provides improvements over these prior processes and methods for the production of zeolitic materials and particularly zeolite A products having improved characteristics over similar products known to the art.